Field
The present disclosure in certain embodiments relates to medium wall seamless steel pipes having a wall thickness (WT) higher than or equal to 6 mm and lower than or equal to 35 mm, and in certain embodiments relates also to a method for manufacturing the seamless steel pipes. Said seamless steel pipes, suitable for linepipes, flowlines and risers for use in the oil and gas industry, can also include pipes that are suitable for hot bending.
Description of the Related Art
Exploration of offshore oil and gas reserves in remote regions of the world is increasingly moving away from conditions where relatively traditional pipe solutions can be utilized and towards more demanding environments. These more demanding environments may incorporate a combination of very challenging factors including, for example, deepwater locations, increased pressure and temperature wells, more corrosive products, and lower design temperatures. These conditions, when added to stringent weldability and toughness criteria already associated with pipe specifications for offshore oil and gas exploration applications, place ever increasing demands on the materials and supply capability.
These demands are evident in project developments involving aggressive environment and high operating pressure. For example, major seamless linepipe manufacturers are able to manufacture pipes of grades X60, X65 and X70 according with American Petroleum Institute (API) 5L and International Organization for Standards (ISO) 3183 standards, with sulfide stress corrosion (SSC) and hydrogen induced cracking (HIC) resistance. However, the strength and toughness trade-off, combined with the need for sulfide stress corrosion (SSC) and hydrogen induced cracking (HIC) resistance (e.g., sour resistance) have been proven difficult to achieve. In particular, quenched and tempered (Q&T) seamless pipes of API 5L grade X60, X65 and X70 typically exhibit maximum hardness values, measured at 1.5-2.0 mm depth from the pipe surfaces (according to API 5L-ISO 3183), below 250 HV10 but however above 235HV10, whereas now new projects require lower values to make the material weldable and resistant to SSC as per Region 3 in NACE MR-0175. These lower maximum hardness values cannot be consistently achieved with current steel chemical compositions and processes.
Moreover, pipelay operations, especially when linepipes are installed by means of the reel laying method, cause repeated plastic bending and straightening deformation cycles. These cyclic loads, producing both cold working and Bauschinger effect, may affect final material stress-strain properties.
Reeling is currently applied to an increasing range of pipe geometries, being the present limit given by pipes with 16″ outside diameter (OD) and 35 mm wall thickness (WT). Other pipeline installation techniques, for example, J-lay, S-lay and steep S-lay also introduce plastic strain.
Accumulated plastic strain, combined with ageing, can reduce the ductility and toughness of the pipe material. Therefore, strain ageing and toughness tests must be carried out following specified procedures when the pipeline is to be exposed to more than 2% plastic strain during installation (as prescribed by DNV OS-F101, 2012). Plastic deformation of the pipe may also decrease the resistance to sulfide stress corrosion cracking, but it is commonly assumed that if traditional safe hardness levels are maintained after straining, there will be sufficient control over the risk of sulfide stress cracking in service.
In the past years, there have been several types of high-strength linepipe steels developed in the field of Q&T seamless pipes. These seamless pipes combine both strength and good girth weldability. However, these seamless pipes exhibit chemical compositions which hinder hardness reduction during tempering. Therefore, close to pipe surface, where very high cooling rates are experienced during external and internal water quenching and high hardness values are obtained after quenching due to formation of a predominant martensitic microstructure, the risk exists that, even after tempering at high temperature and long time, the maximum hardness values along the whole wall thickness of the pipe remain above 235 HV10 and less preferably above 240 HV10.
The alloying concept applied to Q&T seamless pipes of grade X65/X70 by some seamless line pipe manufacturers is based on microalloying with Vanadium and niobium. However, individual hardness values are not systematically below or equal to 235 HV10 along the whole wall thickness.
Moreover, in the case of hot induction bends produced from high strength Q&T seamless pipes, it is more difficult to develop the required grade, combined with good impact toughness and low surface hardness values, while developing concurrently good HIC and SSC resistance. This problem is mainly related to the process conditions used during heat treatment of bends which are necessarily different from those of the seamless pipe. In particular, the quenching process of bends is less effective. This problem cannot be simply solved using steels with higher hardenability (i.e. higher content of chemical elements), because weldability is decreased, toughness is negatively affected, and the risk of hardness peaks is increased.
Examples of manufacturing processes and related steel pipes are disclosed in WO2013/002418, EP1918395A1, US2012/267014A1, US2013/000790A1 and EP1627931A1, the entirety of each is hereby incorporated by reference in their entirety.
WO2013/002418 discloses quenching and tempering conditions adjusted to have yield strength higher than 450 MPa and Vickers hardness HV5 (load of 5 kgf=49 N), measured on the outer and inner side of a pipe, equal or lower than 250 HV5 by carrying out a surface decarburization during heating before quenching and/or a mechanical grinding operation of the as-quenched pipe in order to decrease the surface hardness at the target values. The surface decarburization depth is of 2.5-3.0 mm, while the surface layer mechanically removed is of 0.3-0.7 mm.
EP1918395A1 discloses low carbon steels and a process of manufacturing seamless steel pipes in which, immediately after hot forming the seamless pipe, the pipe is quenched and tempered, or it is put into a holding furnace and soaked at a temperature not lower than Ac3 point, and then quenched and tempered. Therefore the process of EP1918395A1 performs an in-line treatment immediately after hot forming operations, while the pipe is still at temperatures above Ar3 (i.e. without occurring phase transformation from austenite to ferrite). Tempering includes reheating below Ac1 followed by air cooling. Such a process, carried out by using the disclosed low carbon steels, produces grain size numbers defined in the Japanese standard JIS G0551 (1998) that correspond to values of prior austenite grain size (mean lineal intercept, ASTM E112) higher than 32 μm. Disadvantageously these high values of prior austenite grain size (AGS), for these low carbon steels, mean higher steel hardenability with consequently very high hardness values obtained after quenching, whereby, also after tempering, maximum hardness values below 250 HV10 at 1.5-2.0 mm depth from pipe surface cannot be assured. Moreover, coarse AGS leads to poor toughness as impact energy and shear area values are negatively affected by coarse grain population.
US2013/000790A1 discloses that the steel pipe is subjected, immediately after hot rolling and before quenching and tempering, to an optional reheating step S4 (FIG. 5) and/or to an accelerated water cooling step S5 with a cooling rate of at least 100° C./min and a cooling stop temperature of 550° to 450° C. in order to avoid precipitation of carbo-nitrides. After this accelerated-interrupted cooling the pipes have very poor grain growth inhibition. Therefore, coarse austenite grain size (AGS), higher than 25 μm, is expected in these pipes, that means higher steel hardenability with consequently maximum hardness values of 250 HV10 or greater at 1.5 mm depth from pipe surface. Therefore, also poor SSC resistance is expected for these materials close to pipe surfaces.
US2012/267014A1 discloses a process for manufacturing a seamless steel pipe for linepipe capable of improving toughness. The pipe is subjected, after hot rolling and before austenitizing, quenching, and tempering, to an optional reheating step S4 (FIG. 4) and/or to an accelerated water cooling step with a cooling rate of at least 10° C./s. This accelerated water cooling of the hot rolled pipe is stopped when the surface temperature of the pipe reaches at most 450° C. The water cooling stop temperature is preferably at least 300° C., further preferably at least 350° C., and still further preferably at least 400° C., thus the room temperature is not reached. Disadvantageously coarse austenite grain size (AGS), higher than 25 μm, is expected in these pipes. Moreover, the pipes produced with this process are not suitable for severe linepipe applications where resistance to very sour environments and weldability are required.
At last, EP1627931A1 discloses a seamless pipe with a steel composition without Ca and having both V and Nb as mandatory elements, with a (V+Nb) content higher than 0.07%. Disadvantageously this high content of V and Nb contributes to the tempering resistance of bainite and martensite by precipitation hardening, increasing the difficulty in maintaining the maximum hardness below or equal to 235 HV10, close to the outer and inner surfaces of the pipe. Moreover, the control of the shape of inclusions and the enhancement of the HIC resistance by forming fine and substantially round sulfides is not guaranteed due to Ca absence.
Therefore new solutions, which are outside of the conventional pattern for (micro)-alloying additions followed so far for Q&T seamless pipes and hot induction bends, have to be found for high performance X60Q, X65Q and X70Q grade medium wall seamless pipes, with maximum hardness lower than or equal to 235 HV10 and very good impact toughness at low temperature (≦−60° C.).